1. Field of the Invention
The present invention relates to a method for controlling process conditions in an alloy production process, and particularly relates to a method in which a generic process window is defined and a control strategy is implemented based on the defined process window to achieve the desired process conditions.
2. Description of Related Art
It is widely recognized that one of the most important and urgent areas of materials research in the coming decade is the advancement of materials processing technology for a new generation of materials including metals and metal alloys. As an example, eliminating or substantially reducing the material impurities and eliminating or substantially reducing the presence of defects in fabricated parts or components are considered the major bottlenecks in improving the quality of the high performance aircraft engines to be built in this decade and beyond.
Efforts have heretofore concentrated on producing high quality metal powders to be employed in the fabrication of components, and the concentration on production of high quality powders from which components may be made is regarded as a major step in making "clean" materials for parts or components. The production of titanium and/or titanium alloys in powder or ingot form is of special significance in the aircraft engine field, due to the importance of the titanium and its alloys in designing and producing improved engine components. Notwithstanding the effort expended in developing processes or methods to produce high quality metal powders, a serious problem persists with respect to the production of high quality titanium and titanium alloys in that the high level of chemical reactivity of liquid titanium yields or tends to yield unacceptable levels of impurities in the intermediate forms, such as powders, or in the end product.
Because of the high reactivity of liquid titanium, the melting of the titanium or Ti alloy and discharging of the liquid titanium or Ti alloy are generally done in a technique known in the art as cold hearth or skull melting. An example of this technique is described in U.S. Pat. No. 4,654,858, issued to Rowe, and assigned to the assignee of the present application. Other skull melting configurations have also been disclosed in the art, and all of these may be characterized as having a crucible which retains the molten titanium, the crucible being made of a material other than titanium, and, in the "bottom pouring" embodiments, a discharge nozzle, also likely to be made of a material other than titanium. The skull melting technique attempts to avoid the problem of a reaction occurring between the liquid titanium and the crucible and nozzle materials by developing a skull of solid titanium covering the internal surfaces of the crucible and nozzle. The term "continuous skull nozzle process" will be used herein to refer to processes of this type in general.
While continuous skull nozzle processes have been in use in the art for a number of years, problems remain in such processes, particularly those in which an elongated discharge nozzle is employed (as compared with an orifice as depicted in the above-identified '858 patent), in that the formation and control of a stable skull inside the nozzle has proven to be a major hurdle in the development of consistent, dependable processes for melting and discharging the liquid alloy from the crucible. The two principal problems experienced with skull formation in the nozzle are skull "freeze-off" and skull "melt-away". Freeze-off of the skull prevents the continued flow of the liquid alloy out of the crucible to a further apparatus, such as a melt spinning device or continuous ingot casting device. Melt-away of the skull leaves the nozzle material exposed to react with the liquid titanium or alloy, which is likely to cause rapid deterioration of the nozzle by way of either chemical reaction or physical erosion.
Prior attempts to control skull freeze-off or otherwise stabilize the skull geometry in the nozzle have all suffered from disadvantages which have ultimately rendered the proposed solutions ineffective, impractical, and in some instances, undesirable. In one such proposed solution, local induction heating applied to the skull at the nozzle was attempted as a means for preventing nozzle freeze-off from occurring. This approach proved to be ineffective at providing the necessary heat penetration required for maintaining a molten stream at the center of the nozzle, due to the skin effect which concentrates the heat generated at the outer portions of the nozzle and skull. The skin effect of the induction heating actually has a counterproductive effect in that most of the heat generation is concentrated at the outer skin, where a layer of solidified skull is required to be maintained.
The concept of a magnetic levitation nozzle has been propounded as an alternative approach to providing a physical crucible and nozzle structure, thereby eliminating contact between the containment or confinement means and the liquid titanium or alloy thereby preventing any chemical reaction from taking place. Because of the limited strength of the magnetic force, the potential for replacing the skull crucible and nozzle with a levitation nozzle, in view of the current level of technology, shows almost no promise.
The levitation nozzle approach has been proposed for use on a more limited basis to confine the melt stream only. In this approach, an induction coil would be used to confine the melt stream by generating a magnetic field to induce a thin layer of "body force" on the surface of the melt stream, the force having substantially the same effect as creating a positive hydrostatic pressure at the melt stream. The purpose of this type of levitation confinement is to control the flow rate and diameter of the liquid metal melt stream, without specifically dealing with the problem of maintaining a stable skull geometry in the nozzle.
Even in this more limited approach the levitation nozzle is unattractive due to problems intrinsic to the design of the induction coil, and due to problems in the application of this technology to confining the melt stream, such as the alignment of the coil, the stability of the induced current, the electromagnetic field interference and coupling, the complicated coil design, and problems with melt stability, asymmetry and splash. Further, since a crucible and nozzle would still be fundamental components in a system employing levitation to control the diameter of the melt stream, the complicated coupling and interaction between the levitation nozzle and the overall system would require tremendous experimental effort to validate the concept. Simplified experiments are not likely to adequately address the interactions among the levitation force, the nozzle size, and the formation, growth and control of the skull.
Heretofore lacking in prior efforts directed to continuous skull nozzle processes has been systematic investigation of the skull freeze-off and melt-away, which are the serious processing problems in this field. It has further not previously been recognized that a process window for the process of melting and discharging of liquid titanium or other metal or alloy may be developed or defined and used to implement a control strategy in controlling process parameters to produce and maintain a stable skull configuration in the crucible and nozzle.
It is therefore a principal object of the present invention to provide a method for defining a process window for a continuous skull nozzle process which identifies the appropriate conditions for achieving a steady-state solidified layer or skull in a continuous skull nozzle process, and controlling one or more process parameters such that the process operates within the defined process window.
It is another important object of the present invention to provide a method for controlling a continuous skull nozzle process which entails defining a process window for achieving a steady-state solidified layer or skull and using the process window to establish a control strategy whereby the continuous skull nozzle process will be carried out under conditions in which a stable skull configuration exists in the crucible and especially in the nozzle.
It is another important object of the present invention to provide a method for controlling a molten metal flow rate, which subsequently affects a heat transfer rate, the skull thickness, and the melt stream diameter, by use of a pressure differential control.
It is another important object of the present invention to provide a method for controlling a continuous skull nozzle process including the use of a pressure differential control of the molten metal flow rate in combination with other process controls such as control of the superheat temperature in the melt and of the cooling rate in the crucible and nozzle.